Changes in the plant community and ecosystem properties that follow the conversion of agriculture to restored tallgrass prairies are poorly understood. Beginning in 1995, we established a species‐rich, restored prairie chronosequence where ∼3 ha of agricultural land have been converted to tallgrass prairie each year. Our goals were to examine differences in ecosystem properties between these restored prairies and adjacent agricultural fields and to determine changes in, and potential interactions between, the plant community and ecosystem properties that occur over time in the restored prairies. During the summers of 2000–2002, we examined species cover, soil C and N, potential net C and N mineralization, litter mass, soil texture, and bulk density across the 6‐ to 8‐year‐old prairie chronosequence and adjacent agricultural fields in southern Minnesota. We also established experimentally fertilized, watered, and control plots in the prairie chronosequence to examine the degree of nitrogen limitation on aboveground and belowground net primary production (ANPP and BNPP). Large shifts in functional diversity occurred within three growing seasons. First‐year prairies were dominated by annuals and biennials. By the second growing season, perennial native composites had become dominant, followed by a significant shift to warm‐season C4 grasses in prairies ≥3 yr old. Ecosystem properties that changed with the rise of C4 grasses included increased BNPP, litter mass, and C mineralization rates and decreased N mineralization rates. ANPP increased significantly with N fertilization but did not vary between young and old prairies with dramatically different plant community composition. Total soil C and N were not significantly different between prairie and agricultural soils in the depths examined (0–10, 10–20, 20–35, 35–50, 50–65 cm). We compared the results from our species‐rich prairie restoration to published data on ecosystem function in other restored grasslands, such as Conservation Reserve Program (CRP) and old‐field successional sites. Results suggest that rapid changes in functional diversity can have large impacts on ecosystem‐level properties, causing community‐ and system‐level dynamics in species‐rich prairie restorations to converge with those from low‐diversity managed grasslands.

[1] The Mekong River ranks within the top ten rivers of the world in terms of water discharge and sediment load to the ocean, yet its organic matter (OM) composition remains unstudied. This river is experiencing anthropogenically forced changes due to land use and impoundment, and these changes are expected to intensify in the future. Accordingly, we monitored the composition (including vascular-plant signatures) of Mekong River fine particulate organic matter (FPOM) over a one-year period. Autochthonous production comprises a greater proportion of FPOM during the dry season than in the rainy season, as demonstrated by higher percent organic carbon values (7.9 AE 2.4 versus 2.2 AE 0.4%), lower yields of lignin normalized to carbon (0.40 AE 0.05 versus 1.1 AE 0.3 mg (100 mg OC) À1), and an increase in N:C ratios toward phytoplankton values during the dry season (from 0.06 to 0.12). Changes in the lignin-phenol composition of FPOM suggest that gymnosperms contribute more toward FPOM composition during the dry season, with angiosperms dominating in the wet season. This is supported by calculations of the lignin phenol vegetation index of riverine FPOM, which increases between the dry to wet seasons (dry: 29.4 AE 15.0 versus wet: 74.6 AE 17.3). These changes likely reflect seasonal differences in the proportion of flow that is coming from the Upper and Lower Basin, corresponding to compositional differences between the vegetation of these regions. Therefore, this work provides a baseline understanding of FPOM variability that can be used to assess how future change will affect this river.Citation: Ellis, E. E., R. G. Keil, A. E. Ingalls, J. E. Richey, and S. R. Alin (2012), Seasonal variability in the sources of particulate organic matter of the Mekong River as discerned by elemental and lignin analyses,

We examined the factors controlling the variability in water-column respiration rates in Amazonian rivers. Our objectives were to determine the relationship between respiration rates and the in situ concentrations of the size classes of organic carbon (OC), and the biological source (C 3 and C 4 plants and phytoplankton) of organic matter (OM) supporting respiration. Rates were likely elevated in the former rivers, which were all sampled during low water, due to the stimulation of heterotrophic respiration via the supply of a labile, algal-derived substrate and/or the occurrence of autotrophic respiration. The organic composition of fine particulate OM (FPOM) of these rivers is consistent with a phytoplankton origin. Multiple linear regression analysis indicates that [FPOC], C : N FPOC ratios, and [O 2 ] account for a high amount of the variability in respiration rates (r 2 5 0.80). Accordingly, FPOC derived from algal sources is associated with elevated respiration rates. The d 13 C of respiration-derived CO 2 indicates that the role of phytoplankton, C 3 plants, and C 4 grasses in supporting respiration is temporally and spatially variable. Future scaling work is needed to evaluate the significance of phytoplankton production to basin-wide carbon cycling.

The ability to measure the radiocarbon content of compounds isolated from complex mixtures has begun to revolutionize our understanding of carbon transformations on earth. Because samples are often small, each new compound isolation method must be tested for background carbon contamination (C ex ). Here, we present a new method for compound-specific radiocarbon analysis (CSRA) of higher plant-derived lignin phenols. To test for C ex , we compared the ∆ 14C values of unprocessed lignin phenol containing standard materials (woods, leaves, natural vanillin, and synthetic vanillin) with those of lignin phenols liberated by CuO oxidation and purified by twodimensional high-pressure liquid chromatography (HPLC) coupled to mass spectrometry (MS) and UV detection. We assessed C ex associated with (1) microwave assisted CuO oxidation of bulk samples to lignin phenol monomers, (2) HPLC purification, and (3) accelerator mass spectrometry (AMS) sample preparation. The ∆ 14C of purified compounds (corrected for C ex ) agreed, within error, with those of bulk materials for samples that were >10 µg C. This method will allow routine analysis of the ∆ 14 C of lignin phenols isolated from terrestrial, aquatic, and marine settings, revealing the time scale for the processing of one of the single largest components of active organic carbon reservoirs on earth.Higher-plant derived organic carbon represents a significant fraction of the earth's annual primary productivity and is also an important reservoir of fixed carbon, both living and dead. Organic matter (OM) derived from higher plants in terrestrial ecosystems can be processed to soil debris or converted to carbon dioxide through respiration. Once in aquatic ecosystems, such as rivers, more biological processing occurs as plant-derived OM makes its way to the ocean. Once in the ocean, it can be further transformed in the marine dissolved and particulate organic carbon pools, buried in coastal marine sediments, or converted to carbon dioxide. Traditionally, the radiocarbon age of bulk carbon in terrestrial reservoirs is used to infer the average age of mobilized organic carbon. For example, measurements of the ∆ 14 C of bulk particulate organic carbon (POC) and dissolved organic carbon (DOC) suggest that local geology and in situ biological production determine the average age of organic matter in rivers.1-5 Still, terrestrial carbon is a complex mixture that integrates recent biological and ancient geological carbon sources into a complex mixture of carbon-based materials at various stages of decomposition. More in-depth knowledge about the age of various carbon sources can tell us how carbon from different sources is processed. The advent of compound-specific radiocarbon analysis (CSRA) has presented the possibility of measuring the age of individual components within complex mixtures of organic carbon.6,7 Some researchers have used the ∆ 14 C value of long chain fatty acids in river delta sediments to apportion sources of organic matter and, therefore, to understand the fate ...